This dark side of sign tracking may be especiallyworrisome given that the behavior isn’t all that unusual.

Over the years, Robinson has amassed data on close to 2,000

rats. About a third of them exhibit sign-tracking behavior,while a third engage in goal tracking. (The remainingthird vacillate back and forth, and aren’t easily classified asmembers of either group.) In other words, sign tracking —and the potentially problematic behaviors associated with it— appear to be quite common in the rat world. “We’re nottalking about a few extremes,” he says. “We’re actually talkingabout a large proportion of the population.”What makes signs and symbols so powerful for this groupof animals? To answer that question, Robinson has begunlooking more deeply into the brain’s reward system. In astudy last year, he and his colleagues discovered that whensign-trackers glimpse a cue, they experience a rapid spikein dopamine, the neurotransmitter that helps control thebrain’s reward center. Goal-trackers don’t show that spike(Nature, 2011). “One hypothesis is that, for whatever reason,these animals have different brains,” he says. “The behavior ofsign-trackers is strongly controlled by dopamine-dependentmotivational processes. But in the goal trackers, something elseis going on psychologically.”So far, it’s not clear what that something might be.

Robinson’s hypothesis is that for goal-trackers, the cueevokes a more cognitive expectation process that does notrequire dopamine and that leads the animal to the expectedreward. There’s some evidence that the tendency to becomea sign-tracker or a goal-tracker is heritable, Robinson says(Neuropsychopharmacology, 2010). But, he adds, “we knowalmost nothing about the genetic basis.”However, there’s also evidence that early environmentalexperiences can predispose a rat pup to a future of sign- orgoal-tracking. Pups reared in stressful environments withouttheir mothers are more likely to become sign-trackers as adults(Behavioural Brain Research, 2011). “As you’d expect with anycomplex psychological trait, it’s influenced by the interactionbetween environmental and genetic factors,” he says.

Air-popped rewards

To date, most of the work on sign-tracking has been done
in rats. But there are some emerging hints that the same
patterns may show up in people. In a small study of 15 adult
smokers, Stephen Mahler, PhD, at the Medical University of
South Carolina, and Harriet de Wit, PhD, at the University of
Chicago, found that participants who experienced potent cue-induced cravings for cigarettes also reacted more strongly to
food-related images than those who didn’t experience such
cravings (PLoS One, 2010). In humans as in rats, some people
may be highly sensitive to conditioned stimuli.

Margaret Wardle, PhD, a postdoctoral researcher in De Wit’slab, is looking further into sign-tracking by conditioningpeople with food rewards. In her experiment, she is presenting

hungry volunteers with neutral images, such as nature scenes,
on a computer screen. That cue is paired with a reward of
popcorn delivered through plastic tubes. Later, Wardle shows
volunteers a series of images and measures their affinity for
pictures that were previously paired with popcorn.

The research isn’t without its difficulties. “There are some
challenges in moving this over to people,” Wardle says. “People
don’t usually approach or lick or bite things that they’re
attracted to.” Instead, she gauges interest in cues in more
subtle ways. Using sensors attached to the volunteers’ faces, she
measures the activity of barely detectible facial movements.

(The zygomatic muscle involved with smiling, for instance,activates slightly when you’re looking at something you desire.)

Wardle is also tracking reaction time and eye gaze. “Theidea is that sign-trackers will maintain their attention on the[images paired with food], whereas goal-trackers will not,”she says.

This study is just a first pass — “a real rough cut,” Wardle
says — to see whether human sign-trackers can be reliably
identified in lab studies. If she’s successful, it could lead to a
lifetime’s worth of follow-up studies. “My hope is that if we
can identify this trait in a lab, we can start to build a picture
of what a sign-tracking human looks like out in the real
world. How does this relate to vulnerability to drug use?” she
says.

Understanding these behaviors outside the lab will bechallenging, since cues and rewards are often so closely linkedin human life; out in the real world, it’s hard to separate thecocktail glass from the martini it holds. Still, Wardle is hopeful.

“In the future we’d like to do things like look at adolescents,who have comparatively little drug exposure, to see if wecan predict going forward who is going to have what kind ofbehavior,” she says.

That’s precisely where Robinson hopes his animal studies
will lead. “We’re interested in predicting who is going to be
prone to obesity or prone to addiction,” he says. That predictive
power might one day lead to interventions that help prevent
people from developing the maladaptive behaviors in the
first place. The work could also lead to new treatments. Sign-trackers dealing with addiction, for example, might benefit
from therapies that emphasize avoiding cues. “A better
understanding of the neurobiological processes by which
cues gain control over behavior may reveal other treatment
strategies,” Robinson says.

Plenty of dots remain to be connected between lever-gnawing rats and people addicted to food, drugs, alcoholor tobacco. But the research is really taking off, Tomie says.

“There’s a lot of interest right now in the addiction worldabout sign-tracking,” he says. “It’s starting to move from thebackground to the foreground.” n